1. Field of the Invention
The present invention relates to an improved process for the preparation of polyester polyols. More particularly, the invention relates to an improved multistep batch process for the prepartion of polyester polyols by the reaction of a polycarboxylic acid or anhydride with a polyhydric alcohol.
2. Prior Art
The preparation of polyester polyols by the reaction of a polycarboxylic acid or anhydride with a polyhydric alcohol is well known in the art as evidenced by U.S. Pats. Nos. 3,162,616 and 3,716,523. Generally, the processes of the prior art involve a one-step reaction of a polycarboxylic acid or anhydride with a stoichiometric excess amount of a polyhydric alcohol. An excess amount of alcohol is employed to assure that both ends of the polyester terminate in hydroxyl groups, thus providing polyester polyols of low acid numbers, that is, two or less.
One of the problems associated with the prior art processes of preparing polyester polyols from polycarboxylic acids or anhydrides and polyhydric alcohols is the water formed during the esterification, which must be removed from the reaction site. Since for each mole of polycarboxylic acid or anhydride employed in the preparation of the polyol there is produced from 1 to 2 moles of water, the water occupies valuable reactor space, limiting the amounts of reactants which may be initially charged to the reactor. In the normal accepted procedures for the preparation of polyester polyols, all the reactants are charged to a reactor and as the reaction proceeds the water of esterification is removed from the reactor by distillation. Depending on the polyol and the acid or anhydride employed, the amount of water which is removed is approximately 20 to 30% by weight of the total charge. Thus, the final yield of polyol is generally 20 to 30% less than the weight of the charge. The present invention is directed to an improvement in the above-described prior art processes of preparing polyester polyols which allows for an increase in reactor capacity without equipment modification at the same cycle times.